(1) Field of the Invention
The present invention relates generally to communications systems and, more particularly, to a high performance iterative and adaptive equalizer/error correction decoder (turbo-equalization) which is especially suitable for use in underwater telemetry.
(2) Description of the Prior Art
The underwater environment provides numerous difficult obstacles for acoustic communications. The ocean acoustic channel produces large amplitude and phase fluctuations on acoustic signals transmitted therethrough causing temporal, spatial, and frequency dependent fluctuations. Multipath distortion is a significant problem. Underwater regions often experience high and/or variable sound attenuation. Ambient ocean-noise influences the received signal-to-noise ratio and may require high transmission power levels to achieve suitable ratios depending on the conditions.
Presently utilized underwater coherent acoustic telemetry systems are often able to transmit M-ary Phase Shift Keying (MPSK) and M-ary Quadrature Amplitude Modulation (MQAM) signals. At the receiver end, these coherent signals may be processed by an adaptive multi-channel decision feedback equalizer (DFE). The DFE is then usually followed by a de-interleaver and an error correction decoder. The overall performance obtained by this type of prior art underwater telemetry system is often acceptable, but is not satisfactory in many situations. The desire for performance improvement has led to higher performance algorithms whose complexity is orders of magnitude greater than the standard decision feedback equalizer (DFE) system followed by de-interleaving and decoding.
Turbo equalization and turbo coding may be applied to many detection and decoding problems. Turbo coding involves concatenation of simple component codes with an interleaver so that decoding can be performed in steps using algorithms of manageable complexity. However, the complexity of prior art turbo equalization increases exponentially with the number of channels and/or other factors, thereby making a multi-channel telemetry system, as is typically utilized in underwater telemetry systems, highly complex. More particularly, the complexity of the prior art turbo-equalizer grows with channel complexity, modulation level, and spatial and/or time diversity. The complexity of a prior art turbo-equalizer is therefore orders of magnitude greater than the typical DFE structure discussed above.
The following U.S. Patents describe various prior art systems that may be related to the above and/or other telemetry systems:
U.S. Pat. No. 5,301,167, issued Apr. 5, 1994, to Proakis et al., discloses an underwater acoustic communications system that utilizes phase coherent modulation and demodulation in which high data rates are achieved through the use of rapid Doppler removal, a specialized sample timing control technique and decision feedback equalization including feedforward and feedback equalizers. The combined use of these techniques dramatically increases data rates by one and sometimes two orders of magnitude over traditional FSK systems by successfully combating fading and multipath problems associated with a rapidly changing underwater acoustic channel that produce intersymbol interference and makes timing optimization for the sampling of incoming data impossible.
U.S. Pat. No. 5,559,757, issued Sep. 24, 1996, to Catipovic et al., discloses an underwater acoustic telemetry system that uses spatially distributed receivers with aperture sizes from 0.35 to 20 m. Output from each receiver is assigned a quality measure based on the estimated error rate, and the data, weighted by the quality measure, is combined and decoded. The quality measure is derived from a Viterbi error-correction decoder operating on each receiver. The quality estimator exploits the signal and noise differential travel times to individual sensors. The spatial coherence structure of the shallow-water acoustic channel shows relatively low signal coherence at separations as short as 0.35 m. Increasing receiver spacing beyond 5 m offers additional benefits in the presence of impulsive noise and larger scale inhomogeneities in the acoustic field. Diversity combining, even with only two receivers, can lower uncoded error rates by up to several orders of magnitude while providing immunity to transducer jamming or failure.
U.S. Pat. No. 6,295,312 B1, issued Sep. 25, 2001, to Susan M. Jarvis, discloses a method and system for communicating in a time-varying medium. A transmitter sends transmissions of the same message data separated in time with respect to one another. A single sensor receives the transmissions. Each received transmission is buffered until all of the transmissions that were sent are received. The buffered transmissions are simultaneously processed via multichannel adaptive equalization only when all of the transmissions that were sent are received.
The above cited prior art does not disclose a system whose complexity is similar to that of the prior art decision feedback equalizer followed by a de-interleaver and an error correction decoder, but whose performance is comparable to telemetry systems with much higher orders of complexity. The above cited prior art also does not disclose lower complexity, better performing telemetry system which lowers the complexity associated with turbo-equalization. The solutions to the above described and/or related problems have been long sought without success. Consequently, those skilled in the art will appreciate the present invention that addresses the above and other problems.
It is a general purpose of the present invention to provide an improved telemetry system.
An object of the present invention is an improved underwater communication system for coherent signal transmission.
Another object of the present invention is to combine the desirable features of an adaptive feedback equalization (DFE) system with features of a turbo-equalization structure.
Yet another object is to provide an augmented high performance iterative receiver algorithm for underwater acoustic telemetry.
A feature of one embodiment of the invention combines a decision feedback adaptive equalizer (DFE) with a turbo-equalizer whereby the decision feedback equalizer or variant thereof provides a pre-processing stage for a turbo-equalizer that significantly limits the complexity of the turbo-equalizer.
An advantage of the present invention is superior performance as compared to the standard DFE structure.
Another advantage is that time or spatial signal diversity can be processed with low complexity within the DFE to provide a single stream of diversity combined symbols which can be processed with a simplified turbo-equalizer construction for use in multichannel transmissions.
Yet another advantage of the present invention is that a DFE structure may be utilized therein to take advantage of fractional spacing to help synchronize symbols.
Yet another advantage of the present invention is that a DFE structure may be utilized to reduce the extent of the channel response and therefore allow a turbo-equalizer to operate on a much shorter impulse response in order to reduce the complexity thereof.
These and other objects, features, and advantages of the present invention will become apparent from the drawings, the descriptions given herein, and the appended claims. However, it will be understood that above listed objects and advantages of the invention are intended only as an aid in understanding certain aspects of the invention, are not intended to limit the invention in any way, and do not form a comprehensive or exclusive list of objects, features, and advantages.
Accordingly, the present invention provides a coherent receiver which is operable for use in an underwater telemetry system or for other applications. A coherent receiver in accord with the present invention may comprise one or more elements such as, for instance, one or more data input channels connected to the coherent receiver, and/or a decision feedback equalizer for receiving the one or more data input channels and producing a single stream of pre-processed data at a decision feedback equalizer output, and/or a turbo-equalizer connected to the feedback equalizer output for receiving the single stream of preprocessed data.
The coherent receiver may further comprise a plurality of receiver transducers for producing spatially diverse data for the one or more input channels. Alternatively, the coherent receiver may also comprise a single receiver transducer for producing time diverse data for the one or more input channels. In this embodiment, the decision feedback equalizer may also be operable for selectively controlling the total number of the one or more input channels utilized based on error analysis of the diverse data.
The coherent receiver may further comprise a training symbol sequence generator and a correlator in communication with the training symbol sequence generator and the decision feedback equalizer wherein the output of the correlator is preferably receivable by the turbo-equalizer.
In a preferred embodiment, the turbo equalizer may further comprise an equalizer portion and a decoder portion interconnected for iterative operation. The equalizer portion and the decoder portion may each preferably utilize a maximum a posteriori probability (MAP) algorithm.
In operation, the invention comprises a method which may comprise one or more steps such as, for example only, detecting a received signal which may comprise a plurality of data channels, and/or pre-processing the plurality of data channels within a decision feedback equalizer to produce a single output stream of symbol data from the plurality of data channels, and/or post-processing the single output data stream within a single channel turbo-equalizer.
The post-processing of the single output data stream may further comprise iteratively equalizing and decoding data from the single output data stream to produce a corrected data output stream and may utilize the MAP algorithm for the steps of iteratively equalizing and decoding.
In one embodiment, the method may further comprise mitigating phase jitter in the single output data stream utilizing the decision feedback equalizer.
The method may further comprise providing that the received signal comprises a transmitted training symbol sequence, preprocessing the transmitted training symbol sequence to provide a pre-processed training sequence, producing a local training symbol sequence within the receiver, correlating the local training symbol sequence with the pre-processed training sequence to provide channel estimate, and utilizing the channel estimate within the turbo-equalizer.